1. Field of the Invention
This invention generally relates to a lockup device for a torque converter. More specifically, the present invention relates to a lockup device provided with a damper mechanism that is axially movable together with a piston.
2. Background Information
Torque converters usually include a fluid coupling mechanism for transmitting torque between the crankshaft of an engine and the input shaft of an automatic transmission. A torque converter has three types of runners (impeller, turbine, stator) located inside for transmitting the torque by means of an internal hydraulic oil or fluid. The impeller is fixedly coupled to the front cover that receives the input torque from the power input shaft. The hydraulic chamber formed by the impeller shell and the front cover is filled with hydraulic oil. The turbine is disposed opposite the front cover in the hydraulic chamber. When the impeller rotates, the hydraulic oil flows from the impeller to the turbine, and the turbine rotates. As a result, the torque is transmitted from the turbine to the main drive shaft of the transmission.
Generally, a torque converter can perform smooth acceleration and deceleration because it transmits a power via fluid. However, an energy loss occurs due to slip of the fluid, resulting in low fuel consumption.
Accordingly, in recent years to improve fuel efficiency, some of the conventional torque converters have included a lock-up device for mechanically coupling a front cover on an input side and a turbine on an output side. Specifically, the lockup device is disposed in a space between the front cover and the turbine. When the torque converter reaches predetermined operating conditions, the lock-up device of the torque converter causes power from the crankshaft of an engine to be directly transmitted to the automatic transmission, and thus, bypassing the fluid coupling device.
The lock-up clutch primarily includes a disk-like piston a driven plate attached to the power output side of the turbine and a damper mechanism that connects the piston to the driven plate. The piston carries an annular friction member adhered to a position opposed to a flat friction surface of the front cover. The piston is disposed to divide the space between the front cover and the turbine into a first hydraulic chamber on the front cover side and a second hydraulic chamber on the turbine side. As a result, the piston can move close to and away from the front cover by the pressure difference between the first hydraulic chamber and the second hydraulic chamber. When the hydraulic oil in the first hydraulic chamber is drained and the hydraulic pressure in the second hydraulic chamber increases in pressure, the piston moves toward the front cover side. This movement of the piston causes the piston to strongly press against the front cover.
Upon engagement lock-up devices often cause a shudder, or vibration. Further, while engaged, the lock-up device is subject to vibrations caused by sudden acceleration, or deceleration, or other vibration including circumstances associated with internal combustion engines. Consequently, a torsional vibration dampening apparatus is typically employed in lock-up device to dampen vibration.
The damper mechanism of a conventional lockup device includes a drive member fixedly coupled to the piston, a driven member fixedly coupled to the turbine side, and an elastic member (one or more torsion springs) disposed in between the drive member and the driven member to enable torque transmission. The damper mechanism functions as a torsional vibration dampening mechanism to dampen vibration in the lock-up clutch. The drive member is supplied with a torque, e.g., from a clutch coupling portion. The driven member can output the torque to a turbine. The torsion springs elastically couple the drive member and the driven member together in the rotating direction to form a damping mechanism. The damper mechanism is axially movable and acts as a piston of the lockup device. More specifically, the inner peripheral surface of the driven member is non-rotatably engaged with a splined turbine hub for preventing relative rotation and allowing axial movement. As a result, the driven member is non-rotatable and axially movable with respect to the splined turbine hub. The driven member is radially positioned with respect to the turbine hub.
In the conventional lockup device, the operation of the piston is controlled by the working fluid flowing through the main unit of the torque converter. More specifically, a hydraulic operation mechanism in an external position supplies the working fluid to a space between the piston and the front cover when the lockup device is disengaged. This working fluid flows radially outward through the space between the front cover and the piston, and then flows from its radially outer portion into the main unit of the torque converter. When the lockup device is engaged, the working fluid in the space between the front cover and the piston is drained from its radially inner portion so that the piston moves toward the front cover. Thereby, the friction member arranged on the piston is pressed against the friction surface of the front cover. In this manner, the torque of the front cover is transmitted to the turbine via the lockup device.
The conventional lockup device may employ a multi-disk clutch for using multiple friction plates and thereby multiple friction surfaces because only one friction surface cannot provide a sufficient torque transmission capacity in some cases. In this case, one or more plates are disposed between the piston and the front cover.
Certain types of prior art lockup devices are provided with a special annular chamber for supporting the piston. This structure increases the number of parts, and impedes reduction in required space.
Moreover, the lockup device of the torque converter is typically disposed in an axially restricted space within the torque converter.
Meanwhile, the damper mechanism is required to have a higher performance by increasing a transmission torque for operation from a low speed range of a vehicle. In recent years, such a torque converter has been known that the torque transmission via fluid is performed only when starting the vehicle, and the lockup device is engaged in a speed range of 10 Km/h or more. In this structure having an increased lockup range, it has been desired to improve the performance of the torsion springs so that torsional vibrations due to torque variations of an engine can be sufficiently absorbed and damped. More specifically, it has been required to increase the diameter of the torsion spring of the damper mechanism, and thereby improve the characteristics of absorbing and damping the vibrations.
In the conventional lockup device, however, the size of the torsion spring cannot be increased sufficiently, for example, by such a reason that another member is arranged on an axially one side of the torsion spring.
In view of the above, there exists a need for a lockup device which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.